Treatment of metal powder

ABSTRACT

Mixtures of at least two finely divided soft metal powders, e.g., nickel, cobalt, iron and copper powders, are agglomerated by ball milling the powders in a vibratory mill at temperatures between about 40* C. and 200* C. and in a nonoxidizing atmosphere.

United States Patent De Witt Henry West Port Eynon, Swansea;

Alexander Bowen Simpson, Bryn Mill, Swansea; Ross Lowndes Slmms, Swansea; Reginald David Smith, Gorseinon, all oi {72] inventors 75/O.5 BA, 148/126, 264/85, 264/111 [51] int. Cl. B22! 1/00 [50] Field of Search 148/ 1 26; 75/O.5',264/111, 71, 85

[56] References Cited UNITED STATES PATENTS 2,853,767 9/1958 Burkhammer 75/0.5 BA 3,445,555 5/1969 West et a1. 1. 264/111 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorney-Maurice L. Pinel ABSTRACT: Mixtures of at least two finely divided soft metal powders, e.g., nickel, cobalt, iron and copper powders, are agglomerated by ball milling the powders in a vibratory mill at temperatures between about 40 C. and 200 C. and in a nonoxidizing atmosphere.

OF METAL POWDER The present invention relates to milling of mixtures of finely divided, soft metal powders to produce coarser particles and more particularly to' the chemical and mechanical treatment to agglomerate mixtures of soft metal powders of very fine particle size.

Some metal powders are produced in very small particle size. These include copper powder and powders produced by the decompofltion of metal carbonyls, e.g., carbonyl nickel, iron and cobalt powders. Such powders generally have an average particle size of 8 microns or less, for example from 2 to 8 microns according to the conditions of decomposition of the carbonyls or of the production of the copper.

The small particle size and very poor flow properties of such fine powders make them unsuitable for various purposes, in l eluding incorporation into tubular welding electrodes and severely limit the thickness of strip that can be produced from them by roll-compacting. Coarser powders, e.g., with an average particle size of 350 mesh British Standard screen (1388) and above, have much better flow properties, are compatible with the other powder ingredients of tubular welding electrodes and can be roll-compacted to thicker strip, e.g., 0.1 inch thick and above. It is accordingly desirable to increase the average particle size of fine powders, and there is a demand for powder of average particle size in the range of minus 60 to plus 350 mesh 888.

it is already known to increase the average particle size of such powders, and in particular in US. Pat. No. 2,995,780 a process is described in which such powders are increased in average particle size by milling in a high-speed ball mill such as a planetary mill in which the charge and grinding balls are subjected for a period of hours to a high centrifugal field in which the force is at least twice the force exerted by gravity. According to United Kingdom Pat. No. 912,351, the particle size is increased more quickly by performing the milling in an oxygen-free atmosphere, which may be reducing, but is preferably inert, the powder advantageously being reduced chemically to remove any oxide skin already present before being milled. in the processes described in both of these patents the milling takes place at room temperature.

High-speed planetary ball mills are expensive and rather complex pieces of equipment, and are not suitable for largescale production. in addition they can in fact be used only for milling batches of powder and cannot be used in continuous processes.

it has now been discovered that mixtures of finely divided soft metal powders such as nickel, cobalt, iron and copper can be milled in vibratory ball mills under controlled conditions of temperature and atmosphere to agglomerate the finely divided metal powders.

it is an object of the present invention to provide a process for agglomerating soft metal powders in vibratory ball mills.

Another object of the invention is to provide a continuous process for agglomerating finely divided soft metal powders.

Other objects and advantages will become apparent from the following description.

Broadly stated, the present invention contemplates agglomerating mixtures of at least two finely divided soft metal powders by milling in a vibratory ball mill in an atmosphere nonoxidizing to the metal powders being milled and at a temperature of at least about 40 C. to increase the average particle size of the metal powders.

Finely divided, soft metal powders" as used herein refers to powders having an average particle size of less than about 350 mesh B88 and includes sofi metals such as nickel, cobalt, iron and copper. Sofi metal powders produced by vapometallurgical techniques, which ofien have an average particle size of less than about 15 microns are advantageously treated in accordance with the process of the present invention. Thus, the process of the present invention is particularly applicable to the processing of powder mixtures including carbonyl nickel powder of both type A," which consists of discrete particles, and powder mixtures including type B," which consists of interlocking chains of intergrown particles and generally has a low-bulk density.

Advantageously, the atmosphere within the ball mill is reducing to the oxide of the metal being agglomerated. The reducing atmosphere most suitably consists of or contains hydrogen, e.g., at least about 50 percent hydrogen, by volume. The reducing atmosphere can contain inert diluents such as nitrogen but these tend to slow the rate of agglomeration of the metal powders. Advantageously both the gas and the powder are passed continuously through the mill.

The rate of agglomeration increases with temperature. On the other hand, if the temperature is over 200 C. agglomeration of the powder is too severe and the process becomes inoperable. A suitable range of operating temperatures is from 50 C. to 150 C. Within this temperature range the milling time needed depends both on the temperature and on the yield of coarse powder desired, the time taken to bring about a given increase in average particle size decreasing as the temperature increases. It is therefore necessary to correlate the residence time of the powder in the mill with the temperature in order to attain any given particle size. in continuous milling temperatures in the upper part of the range of 50 C. to 150 C. are preferred.

Some examples will now be given. In each case the mill was a vibratory mill of 6.5 liters capacity, and was charged with 13.7 kg. of hardened steel ball bearings (half their number being of 3/4 inch and half of U2 inch diameter), and 2 kg. of the powder under test. Thus, 27 percent of the mill volume was occupied by steel balls.

The processes were conducted in an atmosphere either of pure hydrogen or of a mixture of percent hydrogen and 20 percent nitrogen and other inert gases at a temperature in the range of 90 C. to C. The vibration amplitude was 0.15 inches, the vibration frequency was 1,440 cycles per minute, and the milling time 2 hours.

The powders used had th shown in table 1 below.

properties and compositions W The particle size distribution of various single powders and mixtures milled as described above is given in table 2.

TABLE 2 Particle size distribution in percent, microns Process number Powders atmosphere 250 250450 -75 75-144 44 Comparison of the results of tables 1 and 2 shows that in :ach case the particle size was increased.

The chemical analyses of the products from milling the nixed powders are given in table 3.

TABLE 3 S Size range Example (microns) Percent iron 250-150 0.0' (80% N1; 20% Fe) 32$ 13;; 1o

44 10. 255-150 23. 4 (60% N1; 0% F fifii 3&3 44 32. 5

Percent copper 250 10.8 2150-1150 10. 0 (80% N1; Cu (FN)) 150-75 1%; 44 204 250 30. 4 20 260-150 30. 0 0 (60% Ni; 40% Cu (XFN) 150-75 39.0 75-44 37. l5 44 30. 4 250 17. 0 250-150 17.5 1 (80% N1; 20% Cu (XFN)) 150-75 22.8

44 10. 4 250 40. 7 250-150 41. 0 2 (60% Ni; 40% Cu (XFN)) 150-75 48.8 75-44 09. 0 44 30. 0

It will be seen that all the Nickel-copper mixtures aglomerated to products of constant composition, whereas the Jickel-iron agglomerates varied in composition over the particle size range. In addition the nickel copper agglomerates were bronzelike in color, and could not be separated into their constituents with a magnet, indicating that the agglomerates are not simple mechanical mixtures.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process for agglomerating mixed metal powders which comprises feeding at least two fine metal powders from the group consisting of nickel, cobalt, iron and copper to a vibratory ball mill, establishing an oxygen-free atmosphere in the ball mill, heating the powders to a temperature between about 40 C. and 200 C., and vibrating the ball mill to intimately mix and agglomerate the powders.

2. A process according to claim 1 in which the atmosphere is reducing to the oxides of the metals being milled.

3. A process according to claim 1 in which the atmosphere is hydrogen.

0. A process according to claim 1 in which the atmosphere is passed continuously through the mill.

5. A process according to claim l in which the temperature is between about 50 C. and C,

6. A process according to claim 1 in which the initial particle size of the metal powders being milled does not exceed about 350 mesh B88.

7. A process according to claim 1 in which one of the metal powders is carbonyl nickel powder.

m t l W 

2. A process according to claim 1 in which the atmosphere is reducing to the oxides of the metals being milled.
 3. A process according to claim 1 in which the atmosphere is hydrogen.
 4. A process according to claim 1 in which the atmosphere is passed continuously through the mill.
 5. A process according to claim 1 in which the temperature is between about 50* C. and 150* C.
 6. A process according to claim 1 in which the initial particle size of the metal powders being milled does not exceed about 350 mesh BSS.
 7. A process according to claim 1 in which one of the metal powders is carbonyl nickel powder. 